The present invention relates generally to the field of oil refining and more particularly to a method of pre-treating refinery stock and additives related to that method.
Assume that a gathering line from an oil field delivers a flow of crude oil to a refinery. Prior to treatment in the refinery, including distillation into the various fractions of commercial importance, it is necessary to evaluate the feedstock for metal salts and similar contaminants in the feedstock. If left unchecked, the metal salts typically will accelerate corrosion of the process vessels. With the customary increases in temperature, the metal salts will generate acids which react with the metal surfaces in the process equipment, thereby severely corroding the surfaces of the process equipment, leading to early equipment failure. This mechanism is discussed below. The present disclosure is directed to a reduction in the metal salts. The problem is materially aggravated for crude stocks which have an API gravity of 25 or less. Especially, a crude stock which has an API gravity of about 20 to 25 poses a significant problem. The problem derives in part from the difficulties of separating oil and water where the feed has that range of gravity. Effectively, this relates to the lack of density differences between water and oil.
To provide a bit of background, there are three major metal salts which may be recovered from a producing formation. While they may be in trace quantities, even as few as a few parts per million (ppm hereafter) in the feed will pose a problem. This is especially true of sodium, calcium, and magnesium making up the salts in the flowing feedstock. The presence of some quantity of water may give rise to a water/oil segregation which can in some instances take the metal salts out of the oil. By suitable pretreatment steps, the salt in the oil can be reduced. However, this is more difficult when the oil is very close in density to water. In the past, simply inputting the feed into a large storage tank (or tank farm comprised of many tanks) and waiting for a long interval would tend to drop the water to the bottom. As the water and oil densities become close, there is less likelihood of settling out the water and any water soluble salts that are in it. Therefore, there is a serious problem in removing the salts in crude feedstocks having an API gravity of about 20 to 25.
It is sometimes helpful to add a trace of water, the amount to be discussed, to the flowing crude oil so that the salts can go into solution in the water. The water added will form stable water droplets in the oil. By adding a demulsifier and through the use of high voltage contacts forming an electric field, sometimes the water droplets can be collected and segregated taking advantage of the electric field stress across the flow. This ultimately segregates the water which is then the preferential solvent for the salts and this enables removal of some, perhaps most of the salts in the flow. It is cooperative with a typical wash water added to the heated oil momentarily which comprises about 4% to 8% of the flowing oil volume with a view of removing somewhere between 20% to about 80% of the salt in the crude oil. Interestingly, with high gravity oil, more of the salts can be gotten out because more of the water is taken out, working with a greater density difference between oil and water. If, however, the crude oil has an API gravity of about 20 to 25, removal is degraded, even to as little as 20% of the salt. Leaving 80% of the salt in the crude oil is highly undesirable.
The present disclosure is directed to a method and apparatus for handling that kind of crude and effectively removing far more than just 20% of the salt. Targeting a removal rate of 95% or more of the salts, the present disclosure sets forth a method of pretreatment for the refinery feedstock which assists remarkably in salt removal. It does this by changing the surface tension between the water droplets in the oil, thereby enabling agglomeration of the water. Moreover, the water more readily disperses in the crude. Effectively, the water is more easily collected, thereby converting it more readily from the droplets dispersed through the oil stream. On the one hand, the droplets are highly desirable, thereby yielding a larger oil/water interface for surface contact to thereby preferentially dissolve the metal salts, and yet afterwards, the water is more easily removed thereby taking more of the metal salts with the water. Effectively, the process of the present disclosure overcomes the propensity of metal salts to stay in suspension in the crude oil. They are brought preferentially into the salt water, removed, thereby protecting the downstream equipment from corrosion.
One aspect of the present invention is the injection of a pretreatment mix of water and a special ethoxylated polyol demulsifier with water. The water is added in the range of up to an effective amount being about 1% of the total crude flow. The polyol added is typically in the range of about 5 or 10 ppm; the amount can be increased or decreased dependent on the severity of the problem and the relative API gravity of that particular crude feedstock. As the gravity increases, the amount or the degree of need for the present polyol demulsifier addition is reduced. The method of application will be set forth in detail below. It will be given in the context of an operating crude oil processing unit typically incorporating a distillation column for breaking down the crude into the various cuts or subsequent use. Further, the context will provide a method of use and will also provide a method of manufacture of the ethoxylated polyol for the present disclosure.
Since the filing of my U.S. patent application Ser. No. 09/260,447 filed Mar. 2, 1999 now U.S. Pat. No. 6,086,750. I have also discovered that the addition of caustic to process provides three additional benefits: (1) the caustic help to water-wet the solid crystalline salts or inorganic materials; (2) the caustic also forms metal hydroxides with other contaminants in the oil, making them more water soluble and thus more easily removed from the oil; and (3) the caustic greater enhances the breakout of water from the oil. I have found that caustic in the pH range of 7-12, and preferably in the pH range of 9-12, in addition to the water and other additives of my method, provide significant enhancement of the benefits of my method.